Method of etching refractory metal based materials uniformly along a surface



Nov. 19, 1968 H. P. WEINBERG 3,411,999

METHOD OF ETCHING REFRACTORY METAL BASED MATERIALS UNIFORMLY ALONG A SURFACE Filed Dec. 10, 1965 FIG. I

I N POWER E5\ OSCILATOR M m III FIG. 4A H63 INVENTOR I r\ Will/[111M I i E? v United States Patent 3,411,999 METHOD OF ETCHING REFRACTORY METAL BASED MATERIALS UNIFORMLY ALONG A SURFACE Harold P. Weinberg, Alexandria, Va., assignor to Value Engineering Company, Alexandria, Va., a corporation of Delaware Filed Dec. 10, 1965, Ser. No. 512,935 Claims. (Cl. 204-141) The present invention relates to the removal of metal from a surface of a metallic article with an acid, more particularly to a method and apparatus 'forchemical etching and electrochemical etching of articles of refractory metals and refractory metal based alloys, and carbides.

The processes of chemical etching and electrochemical etching have both been used in industry to remove metal from selected surface areas of metallic articles. These etching processes have the advantage that they do not affect the base material in the same manner as mechanical metal-removing processes such as machining. However, chemical etching and electrochemical etching have the disadvantages that they are relatively slow processes in that they have a slow rate of attack and a non-uniform rate of attack with the result that metal is not removed at uniform depths in .a selected area. Chemical etching is further characterized by the accelerated attack at the grain boundaries to produce unsatisfactory surface finishes. The relatively slow rate of attack in electrochemical etching is believed to be caused by an insulating dissolved ion layer formed around the article being worked.

These problems inherent in chemical etching and electrochemical etching are particularly acute when working refractory metals and alloys such as aluminum, titanium, beryllium, tungsten and carbides. Further, these processes produce unsatisfactory surface finishes in these metals.

In chemical etching the rate of attack and the uniformity of the attack are largely controlled by the etchant employed and the nature of the metal being worked. Thus, these parameters are not controlled by outside forces or factors. In electrochemical etching there is the added parameter of electric current. An increase in current will increase the rate of attack but any significant increase in the rate of attack requires a considerable increase in the electric current. Such an increase in current becomes uneconomical and, hence, it is not feasible merely to increase the current to accelerate the process.

Numerous efforts have been made to improve the processes of chemical etching and electrochemical etching by improving the etchants and electrolytes and by varying the current densities and variation of the etching currents. However, none of these efforts have produced any significant improvements in these processes.

It is therefore the principal object of the present invention to provide a novel and improved method and apparatus for chemical etching and electrochemical etching of metals, particularly aluminum, tungsten, titanium, beryllium and carbides.

It is another object of the present invention to provide an efficient method and apparatus for chemical etching and electrochemical etching without deleterious effects such as uneven attack.

It is a further object of the present invention to provide a method and apparatus for chemical etching and electrochemical etching which provides a significant increase in the rate of attack.

It is an additional object of the present invention to provide a method and apparatus for improving chemical etching and electrochemical etching processes by an external force or factor.

The present invention essentially comprises surrounding 3,411,999 Patented Nov. 19, 1968 the metallic article to be worked with ultrasonic vibrations to form a sheath or envelope around the article. The article itself is not vibrated but ultrasonic vibrations are introduced into the etchant or electrolyte and then uniformly dispersed within the liquid :around the article.

The method of the present invention is carried out by initially immersing the metallic article to be worked within an etchant for chemical etching or an electrolyte in electrochemical etching. The volume of the etchant is then positioned upon a body of water. Ultrasonic vibrations are then produced in the water and are transmitted through the water to the etchant or electrolyte. The water thereby acts as a water couple. The ultrasonic vibrations are then uniformly distributed within the etchant or electrolyte around the metallic article to form a sheath surrounding the article. This is accomplished by interposing a rigid diaphragm of a hard brittle material between the water and the etchant to prevent contact therebetween. The article itself is not vibrated but is suspended within the etchant or electrolyte in the conventional manner so that the selected surface to be worked is submerged in the solution. In electrochemical etching the metallic article is connected to a source of electrical energy and is made the anode. A cathode, also connected to the source of electrical energy, is positioned within the electrolyte.

The apparatus comprising the present invention includes a tank for containing Water. A container of a hard brittle material, such as glass, is then positioned within the water. The etchant or electrolyte is contained within this glass tank. A stainless steel diaphrgam is then provided in the bottom of the water tank and a transducer for ultrasonic vibrations is then mounted on the outer surface of the stainless steel diaphragm. The transducer is connected to an oscillator so that the ultrasonic vibrations provided by the transducer are distributed through this steel diaphragm into the water. The metallic article to be worked is then immersed into the etchant or electrolyte. The ultrasonic vibrations are then transmitted through the water acting as a water couple to the brittle container with the etchant. The etchant container acts as a brittle membrane and uniformly distributes the ultrasonic vibrations throughout the etchant. These distributed ultrasonic vibrations form a sheath or envelope around the metallic article. It has been found that uniformly distributing the ultrasonic vibrations within the etchant or electrolyte to surround the metallic article significantly increases the rate of attack and also enables the etchant or electrolyte to attack uniformly the selected surface of the metal article.

Articles to be worked may be in the form of wires or filaments and the process can be carried out continuously or as a batch process.

Other objects and advantages of the present invention will be apparent from the accompanying description when taken in conjunction with the following drawings wherein:

FIGURE 1 is an overall perspective view of the apparatus of the present invention as used for chemical etching;

FIGURE 2 is a vertical sectional view taken along the line 22 of FIGURE 1;

FIGURE 3 is a sectional view similar to that of FIG- URE 2 but showing the arrangements for electrochemical etching;

FIGURE 4A is a vertical sectional view through a metallic article having a resist and showing th lack of uniform attack in electrochemical etching because of the dissolved ion layer and without employing the teachings of the present invention; and

FIGURE 43 is a sectional view similar to that of FIGURE 4A and illustrating the uniform attack on the same metallic article when using the ultrasonic vibrations according to the present invention.

Proceeding next to the drawings wherein like reference symbols indicate the same parts throughout the various views, specific embodiments of both the method and apparatus of the present invention will be described in detail.

The apparatus as illustrated in FIGURE 1 comprises a stainless steel tank 1 which is substantially filled with water 2 at room temperature. Supported on the upper edges of the tank 1 is a glass container 3 for the etchant 4. The container 3 is formed of glass such as Pyrex to form a brittle hard membrane for transmitting ultrasonic vibrations to the etching solution 4. The container 3 has flanges 5 and 6 for supporting the container on the tank 1. The precise nature of' the etching solution 4 will vary but generally an aqueous solution of hydrofluoric acid is used.

At the bottom of the tank 1 there is provided a stainless steel diaphram 7. Mounted immediately below the stainless steel diaphram is a barium titanate ceramic transducer 8.

The transducer 8 is entirely conventional and is powered by a coaxial cable 9 connected to a power oscillator 10. The transducer 8 includes a bakelite housing 11 which supports a transducing element 12 which is generally in the shape of a flat plate and formed of barium titanate ceramic. The coaxial cable 9 is coupled through a conventional connector 13 to the transducer element by the wires 14 and 15.

The article to be etched is indicated at 16 and is immersed within the etching solution 4. The article 16 is of a metal such as titanium and the surfaces which are not to be subjected to the chemical etching process of this invention are covered with a resist or mask 17 of a chemically resistant strippable coating. The selected surface of the metallic article which is to be etched is indicated at 18.

The article 16 is gripped by platinum pincers 19 which are mounted on the lower end of a supporting structure indicated at 20.

For electrochemical etching a metallic article the apparatus employed is illustrated in FIGURE 3 and is similar in all respects to that of FIGURES l and 2 except for the addition of the cathode and source of electric current. In FIGURE 3 the platinum pincers 19 are mounted on the lower end of a copper rod 21 which is supported from a frame structure 22. The pincers 19 are biased by a spring 23 to grip the metallic article 16. The supporting frame 22 also supports an electrode 24 preferably of high purity graphite which projects downwardly within the container 3. The copper rod 21 is connected to the positive terminal of a DC power supply 25 and the electrode 24 is connected to the negative terminal of that supply.

In carrying out the new and improved method of chemically etching a metallic article according to the present invention a metallic plate formed of a 6061-T6 aluminum alloy is immersed in an etching solution comprising of 1% hydrofluoric aqueous solution. The etchant which is in the glass container 3 is then suspended into a body of water 2 in the tank 1. The water may be ordinary tap water and is at room temperature. The power oscillator is then energized to produce ultrasonic vibrations in th transducer at a frequency of kc. It has been found that the exact frequency of the ultrasonic vibrations is not critical but the vibrations must be ultrasonic. Different frequencies can be used depending upon other factors including the nature of the electrolyte and the metal being worked.

With the ultrasonic vibrations being produced at a distance from the etchant 4 the vibrations are then transmitted through the water 2 which acts as a water couple. The transmitted vibrations are then uniformly distributed throughout the etchant by the action of the brittle membrane of the glass container 3. These vibrations are thus uniformly distributed throughout the etchant so as to form a sheath or envelope around the metallic article during the action of the etchant upon the article.

The rate of metal removal with the ultrasonic sheath surrounding the aluminum article was determined to be 0.0015 inch per minute, whereas without the ultrasonics the rate of metal removal was only 0.0005 inch per minute.

It is believed that the ultrasonic vibrations surrounding the metallic article within the etchant increased the rate of metal removal by causing migration of the dissolved ions away from the surface of the article which is being attached by the etchant. Also, these vibrations force the gaseous reaction products of the etchant and the aluminum article, i.e., hydrogen, into cavitation at the working surface to produce a force on the surface of the article when the gaseous bubble collapses. Further investigation is being conducted to determine the precise causes of this significant increase in the rate of metal removal.

Not only is the rate of metal removal increased but the attack of the etchant upon the exposed surface of the article becomes uniform. Reference to FIGURE 4A will show that without ultrasonics the etchant attacks the edges of the exposed surface at a greater rate than the center of the exposed surface to form the cross-section as illustrated in this figure. It is believed that this lack of uniform attack is due to the presence of the dissolved ion layer on the exposed metal surface. However, with the use of ultrasonics according to the present invention the selected surface of the metal is attacked uniformly as illustrated in FIGURE 4B. Not only is the rate of attack uniform but the finish is considerably improved. The ultrasonic vibrations adjacent to the surface of the metallic article apparently caused the dissolved ions to dissipate from the surface and thus retard the formation of the gaseous products. As a result, the reaction of the etchant upon the exposed metal proceeds much more rapidly and smoothly.

Further, the action of the etchant upon the exposed metal does not produce any undercut beneath edges of the resist as shown in FIGURES 4A and 4B.

It has been found that when the article itself is vibrated node points developed upon the surface of the article. This caused uneven metal removal. However, when the vibrations are uniformly distributed throughout the etchant no node points whatsoever are formed on the surface of the metal article, and accordingly, the rate of metal removal is uniform.

In order to carry out the new method of electrochemically etching an aluminum article according to the present invention, the apparatus as illustrated in FIGURE '3 can be used. In both chemical and electrochemical etching of aluminum it is normal for a sludge to form at the surface of the aluminum. This necessitates the use of sequestering agents. A further limitation in the chemical etching of aluminum is the formation of hydrogen bubbles at the metal-liquid interface which thus interferes with the etching acid. When aluminum is subjected to electrochemical etching, the sludge and bubble problems still exist but a more serious problem is the formation of an insulating ion-barrier at the metal-liquid interface.

According to the present invention an article formed of 6061-T6 aluminum alloy was immersed in an aqueous electrolyte etchant. A stainless steel cathode was used. The aluminum article thus acted as the anode and both the anode and cathode were connected to a source of electrical energy so that the aluminum article was subjected to a current density of approximately amp. per square foot. Without the application of ultrasonics the average rate of metal removal was 0.0003 inch per two minutes.

According to the present invention ultrasonic vibrations at a frequency of 25 kc. were then produced in the transducer and transmitted through the water couple 2 to the brittle membrane of the electrolyte container 3. The brittle membrane then uniformly distributed the ultrasonic vibrations throughout the electrolyte and around the aluminum article to form a sheath therearound. Under the same conditions, but applying the above ultrasonic vibrations, the rate of metal removal averaged 0.009 inch per two minutes. Not only was the rate of metal removal considerably increased but the surface finish was greatly improved since the exposed metal was uniformly attacked by the electrolyte. The etched surface of aluminum without ultrasonic vibrations was examined under magnification of 50X, 100x, and 200x and then after ultrasonics were applied was similarly examined under the same magnifications. The more uniform attack of the surface by the electrolyte was clearly apparent under these magnifications. It is therefore apparent that the use of ultrasonic vibrations as taught by the present invention negated the effects of the adherent sludge and the formation of the insulating layer of dissolved ions.

The use of the brittle membrane in this process is critical since this membrane has been found to produce an even distribution or evening out out of the ultrasonic vibrations within the etchant or electrolyte. Without the use of the brittle membrane the ultrasonic vibrations within the etchant have a tendency to form node points or areas of high concentration in the etchant solution at distances which are an increment of the natural frequency of the diaphram. As a result, the exposed surface of the metal article is etched in patches and the result is an unsatisfactory finish.

Further examples of chemical etching of other materials according to the present invention are as follows:

EXAMPLE 2 Titanium alloy was immersed in an etchant consisting of a 5% HF-aqueous solution. Without ultrasonic vibrations the rate of metal removal was 0.0002 inch per minute and the surface finish was 63 (microinches). With the ultrasonic vibrations the rate of metal removal increased to 0.0008 inch per minute and the surface finish was 16 (microinches) EXAMPLE 3 A beryllium sheet was immersed in an etchant consisting of:

53 gm. chromic anhydride 26.5 cc. conc. H 80 450 cc. orthophosphoric acid Without ultrasonic vibrations the rate of metal removal was 0.0002 inch per minute and the resulting surface finish was With ultrasonic vibrations the rate of metal removal increased to 0.0009 inch per minute and the resulting surface finish was EXAMPLE 4 A sheet of 5086 1-1112 aluminum alloy was immersed in an etchant consisting of NaOH aqueous solution. Without ultransonic vibrations the rate of metal removal was 0.0005 inch per minute. With ultrasonic vibrations the rate of metal removal was 0.003 inch per minute.

EXAMPLE 5 A titanium article was immersed in an etchant of HF 60 mils/l. and HNO 230 mils/l. and at a temperature of 115 F. Without ultrasonic vibrations the rate of metal removal was 0.5 mil per minute, but with ultrasonic vibrations was 1.2. mils per minute.

With respect to the electrochemical etching of titanium, the following example under the set forth conditions were carried out:

EXAMPLE 6 A titanium article was immersed in an electrolyte of a 10% HF aqueous solution. A graphite cathode was used and the current density was 200 amps/fe The rate of metal removal without ultrasonic vibrations was 0.0025 inch per minute but with ultrasonic vibrations was 0.012 inch per minute.

In order to determine whether the attack on titanium would be rough and irregular if the titanium were etched with greater depth by the ultrasonic process according to the present invention, several samples of titanium were run and approximately of an inch removed by the present ultrasonic process. These samples were then crosssectioned and photomicrographs taken on the magnifications of 20X, x, and 500x. The titanium was subjected to a current density of approximately 200 amps per square foot for 15 minutes. These photomicrographs clearly showed that the attack was smooth and regular and further demonstrated that the rate of metal removal could be easily accelerated according to the present inventi-on without affecting the finish of the titanium.

The following example is electrochemical etching of tungsten:

EXAMPLE 7 EXAMPLE 8 Sheet beryllium was immersed in an electrolyte of:

Parts phosphoric acid 100 sulfuric acid 30 glycerol 30 absolute ethenol 30 A stainless steel cathode was used and the current density was 10 amps/ftP. The rate of metal removal without ultrasonic vibrations was 0.0002 inch per minute but with ultrasonic vibrations was 0.0012 inch per minute.

The invention as described above has been essentially a batch process wherein metallic articles or components were subjected to chemical etching or electrochemical etching. However, the present invention can also be applied continuously to metallic wires or sheets.

One continuous application of the present invention has been in the finishing of beryllium wire. In order that such Wire exhibit maximum properties the wire should have a particularly high degree of smoothness and cleanliness. However, this finishing process of the wire must be carried out by an economical efficient metal removal process which does not introduce deleterious effects to the wire. Such effects have been experienced in previous chemical etching processes.

The drawn beryllium wire as received contained a large quantity of surface blemishes and lubricating compound. It is highly desirable to etch the surface of this Wire to obtain a smooth finish. To date, however, the usual etching processes have not produced satisfactory surface finishes.

The surface blemishes and irregularities of unetched beryllium wire are apparent from a series of photomicrographs taken of the wire under magnifications of 50X, 100x, 200x, and 400x. Cross-sections of the same unetched wire were also examined in photomicrographs taken under magnifications of 200x and 400x.

It was found that the regular chemical etching of beryllium wire using various known etching solutions produced the normal irregular attack on the surface of the wire. This was clearly evident from a cross-section photomicrograph taken under a magnification of 400x.

EXAMPLE 9 Beryllium wire having an CD. of 0.005 inch was subjected to a chemical etch by moving the wire continuously through an etchant consisting of a chromic acidphosphoric acid aqueous solution. The wire was immersed in the etchant for 1 minute. Without ultrasonic vibrations the amount of metal removed was not measurable. However with ultrasonic vibrations 0.0015 inch of metal removed and much smoother surface finish obtained.

EXAMPLE Beryllium wire having a 0.005 inch CD was subjected to an electro-chemical etch 'by moving the wi re' through an electrolyte of 0.1 N nitric acid. The wire .was immersed for seconds, the voltageapplied was 3;v olts and a stainless steel cath-ode used. Without ultrasonic vibrations 0.001 inch O.D. removed but the wire hada very ragged surface. With pltrasgnig vibrations 0.002 inch O.D. removed but there was also atremendous dif ference in the mode of attack which resulted in a smooth surface finish.

- .E M LEU. A nickeLcoated beryllium wire having an of 0.006 inch'was immersed for 2-5-30"seconds in an electrolyte of 3-4 N nitric; acid. A-'v0ltag'e off 5 volts wasapplied and a stainless steel cathode use'dJWith ultrasonic vibrations the wire was reduced to an OD. of 0.004 inch and the nickel was completely removed. p

The present invention has also been used tofinish a boron wire filament to a smooth surface in order to achieve optimum physical and mechanical properties. Heretofore this could not be done because of the limiting parameters of the available processes for finishing such wire.

The boron filament as manufactured has a high surface roughness which requires finishing operation. When boron wire having a 0.005 inch CD. is viewed under a magnification of 50X, the high surface roughness is readily apparent.

The boron Wire filament having a 0.005 inch CD. was subjected to a chemical etch without ultrasonic vibrations using HNO as an etchant for a period of four minutes. Examination of the boron filament under a magnification of 150 showed a relative lack of attack and the slight attack existed only at the grain boundaries.

subjecting the same boron filament to the chemical etch for minutes and then viewing the boron filament under a magnification of 200 revealed that the attack had become heavier but was still concentrated in the grain boun daries.

Electrochemically etching the same boron filament in the same etchant for minutes without ultrasonic vibrations with the filament as the anode showed the same attack. Here the filament was viewed under a magnification of 400x.

The process of the present invention which included ultrasonic vibrations introduced into the etchant in order to form a sheath around the boron wire filament was then carried out. Nitric acid (1 N) was used as the electrolyte and a potential of 5 volts was applied to the filament as the anode for 5 minutes. Photomicrographs were then taken of the electrochemically etched boron wire filament under magnifications of 100x, 150x, 200x and 400x. These photomicrographs clearly showed that the grain of the boron was almost completely removed and smoothed out. The result was a boron wire filament having a smooth surface but whose mechanical and physical properties were unimpaired.

Thus it can be seen that the present invention has provided a method and apparatus which can be used in either batch or continuous treatment of various metals in order to give these metals a smooth surface finish without impairing their mechanical or physical properties. Further, the economic feasibility of chemical etching and electro chemical etching of refractory metals has thereby been significantly improved by the increased rate of attack and the uniform attack provided by the present invention. It will be understood that this invention issusceptible to modifications in order to adapt it to difi'erent usages and conditions, and accordingly, it is desired to compre hend such modifications within this invention as maygfall; 7

Within th e scope of the appended claims. Wh is cl ime i v1. Amethod of chemicallyietching anarticle ofrefrac tory metals, refractory metal based alloys and;carbides to removemetal'at substantially a, uniform rate along a sur-j facethereof, and, comprising-the steps of immersingthe article into :a volumeof an. etjchant-, .proyidingjabody of liquid to surround a: substantial portion .of. said etchant and interposing a rigid. diaphragm between. said-liquid and .saidi-etchant toprevent contact between saidliq'uid and said "etchant;producing:ultfaSoriicrVibTations in the body' of liquid with, said..'vibrations..be'ing transmitted through the:liqiuid anddiaphragm to the.etcharit and uni-z formly distributed Within the .etchant around the immersed portion'of the article. 1.. 1 1 V 2. A: method of chemically :etchingametallic article as claimed. in claim 1 wherein the liquid is water. u.

3. A method of chemically etching ametallic article as claimed in claim l wherein the ultrasonic vibrations with; in the .etchant are formed into a sheath around the im-: mersed portion of the metallic article. I 4. In the'method of chemically etching metallic articles as claimed in claim 1 wherein the article is formed of one of titanium, tungsten, beryllium and aluminum.

5. A method of chemically etching a metallic article as claimed in claim 1 wherein the article consists of a wire of one of boron and beryllium.

6. A method of electrochemically etching an article of refractory metals, refractory metal based alloys and carbides to remove metal at substantially a uniform rate along a surface thereof, and comprising the steps of providing a body of liquid to surround a substantial portion of said etchant and interposing a rigid diaphragm between said liquid and said etchant to prevent contact between said liquid and said etchant, bringing a portion of an article into contact with the etchant, producing ultrasonic vibrations within the body of liquid with said vibrations being transmitted through the body of liquid and diaphragm to the etchant and uniformly distributed within the etchant around the immersed portion of the article and polarizing the article anodically to pass an electric current from said article through the electrolytic chemical etchant.

7. A method of electrochemically etching a metallic article as claimed in claim 6 wherein the liquid is water.

8. A method of electrochemically etching a metallic article as claimed in claim 6 wherein the ultrasonic vibrations within the etchant are formed into a sheath around the immersed portion of the metallic article.

9. In the method of electrochemically etching metallic articles as claimed in claim 6 wherein the article is formed of one of titanium, tungsten, beryllium and aluminum.

10. A method of electrochemically etching a metallic article as claimed in claim 6 wherein the article consists of a wire of one of boron and beryllium.

- References Cited UNITED STATES PATETNS 2,744,860 5/1956 Rines 204-45 2,861,932 11/1958 Pohl 204-141 2,876,083 3/1959 Prietl 204-157.15 7 3,066,089 11/1962 Davies 221-81 ROBERT K. MIHALEK, Primary Examiner. 

1. A METHOD OF CHEMICALLY ETCHING AN ARTICLE OF REFRACTORY METALS, REFRACTORY METAL BASED ALLOYS AND CARBIDES TO REMOVE METAL AT SUBSTANTIALLY A UNIFORM RATE ALONG A SURFACE THEREOF, AND COMPRISING THE STEPS OF IMMERSING THE ARTICLE INTO A VOLUME OF AN ETCHANT, PROVIDING A BODY OF LIQUID TO SURROUND A SUBSTANTIAL PORTION OF SAID ETCHANT AND INTERPOSING A RIGID DIAPHRAGM BETWEEN SAID LIQUID AND SAID ETCHANT TO PREVENT CONTACT BETWEEN SAID LIQUID AND SAID ETCHANT, PRODUCING ULTRASONIC VIBRATIONS IN THE BODY OF LIQUID WITH SAID VIBRATIONS BEING TRANSMITTED THROUGH THE LIQUID AND DIAPHRAGM TO THE ETCHANT AND UNIFORMLY DISTRIBUTED WITHIN THE ETCHANT AROUND THE IMMERSED PORTION OF THE ARTICLE.
 6. A METHOD OF ELECTROCHEMICALLY ETCHING AN ARTICLE OF REFRACTORY METALS, REFRACTORY METAL BASED ALLOYS AND CARBIDES TO REMOVE METAL AT SUBSTANTIALLY A UNIFORM RATE ALONG A SURFACE THEREOF, AND COMPRISING THE STEPS OF PROVIDING A BODY OF LIQUID TO SURROUND A SUBSTANTIAL PORTION OF SAID ETCHANT AND INTERPOSING A RIGID DIAPHRAGM BETWEEN 